[fix] Travis build for NDK 15 (#512)
[fix] Travis build for NDK 15 (#512)

#510 passed even though it should've failed.
koreader/koreader-base,koreader/koreader-base,apletnev/koreader-base,NiLuJe/koreader-base,Frenzie/koreader-base,houqp/koreader-base,Frenzie/koreader-base,apletnev/koreader-base,houqp/koreader-base,apletnev/koreader-base,koreader/koreader-base,NiLuJe/koreader-base,apletnev/koreader-base,NiLuJe/koreader-base,houqp/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,Frenzie/koreader-base,houqp/koreader-base,koreader/koreader-base